I. Introduction

In the electric drive of chemical enterprises, the application of variable frequency drive of centrifuge is very common. Due to various reasons of process and drive equipment, the phenomenon of regenerative energy often occurs. In general frequency converters, there are two most commonly used methods for processing regenerative energy. : (1) Dissipate into the "brake resistance" artificially set in parallel with the capacitor in the DC circuit, which is called the dynamic braking state; (2) Make it feed back to the grid, it is called the feedback braking state ( Also known as regenerative braking state). The principle of the DC common bus is based on that the universal frequency conversion device adopts the AC-DC-AC frequency conversion method. When the motor is in the braking state, its braking energy is fed back to the DC side. In order to better handle the feedback braking energy, people adopt The way to connect the DC side of each inverter device. For example, when one inverter is braking and another inverter is accelerating, the energy can be complementary. This paper proposes a scheme for a common frequency converter to share the DC bus in the centrifuge of a chemical enterprise, and explains its further application in the feedback unit of the centrifuge.

At present, there are many ways of DC common bus:

(1) Share an independent rectifier The rectifier unit can be non-invertible or reversible. The former energy is consumed by an external braking resistor, and the latter can fully feed the excess energy on the DC bus directly to the grid, which has better energy saving and environmental protection significance. The disadvantage is that the price is higher than the former.

(2) The large inverter unit is connected to the power grid. The small inverter shares the DC bus of the large inverter. The small inverter does not need to be connected to the power grid, so there is no need for a rectifier module. The large inverter is externally connected with a braking resistor.

(3) Each variable frequency unit is connected to the power grid. Each variable frequency unit has a rectifier, inverter circuit and an external braking resistor, and the DC bus is connected to each other. This situation is mostly used when the power of each frequency conversion unit is close. After disintegration, it can be used independently without affecting each other. The DC common bus introduced in this article is the third method, which has great advantages over the first two:

a. Sharing the DC bus can greatly reduce the repeated configuration of the braking unit, the structure is simple and reasonable, economical and reliable.

b. The intermediate DC voltage of the shared DC bus is constant, and the parallel connection of capacitors has a large energy storage capacity, which can reduce the fluctuation of the power grid.

c. Each motor works in different states, and the energy feedback is complementary, which optimizes the dynamic characteristics of the system.

d. The different sub-harmonic interference generated by each frequency converter in the power grid can cancel each other, reducing the harmonic distortion rate of the power grid.

2. The frequency conversion speed regulation system scheme before the transformation

2.1 Introduction to the centrifuge control system

There are a total of 12 modified centrifuges, and each control system is the same. The frequency converter is Emerson EV2000 series 22kW, constant torque type, the feedback units are all IPC-PF-1S regenerative braking units with energy, and all control systems are concentrated with eight units similar to this. The system diagram is shown in Figure 1.

Figure 1 Schematic diagram of the inverter and braking unit system before the transformation

It can be seen from Figure 1 that each inverter needs a feedback braking unit, and their respective control systems are completely independent.

2.2 Analysis of braking work when braking

When the centrifuge brakes, the motor will be in a regenerative braking state. The mechanical energy stored in the system is converted into electrical energy by the motor and sent back to the DC circuit of the frequency converter through the six freewheeling diodes of the inverter. At this time, the inverter is in a rectifying state. At this time, if energy consumption measures are not taken in the frequency converter, this part of the energy will cause the voltage of the energy storage capacitor of the intermediate circuit to rise. At this time, the DC bus voltage of the capacitor rises. When it rises to 680V, the braking unit starts to work. That is, the excess power is fed back to the grid side. At this time, the DC bus voltage of a single inverter is maintained below 680V (some 690V), and the inverter will not report an overvoltage fault. The current curve of the braking unit of the inverter when braking is shown in Figure 2. The braking time is 3 minutes. The testing instrument is FLUKE 43B single-phase power quality analyzer, and the analysis software is "FlukeView Power Quality Analyzer Version 3.10.1".

Figure 2 Current curve of the brake unit when it is working

It can be seen that each time the brake is applied, the brake unit must work, with a maximum current of 27A. The rated current of the braking unit is 45A. Obviously the brake unit is in a half-loaded state.

3. The frequency conversion speed regulation system scheme after transformation

3.1 Disposal method of common DC bus

It is very important to use the shared DC bus that the control of the inverter, transmission failure, load characteristics and maintenance of the input main circuit must be fully considered when powering on. The program includes 3-phase incoming lines (maintain the same phase), DC bus, general inverter group, common braking unit or energy feedback device and some auxiliary components. For general-purpose inverters, Figure 3 shows one of the more widely used solutions. The main circuit system diagram after the third modification scheme is selected is shown in Figure 3. In Figure 3, the air switches Q1 to Q4 are the incoming line protection devices of each inverter, and KM1 to KM4 are the power-on contactors of each inverter. KMZ1 to KMZ3 are parallel contactors of the DC bus. 1#, 2# centrifuges share a braking unit to form a group, 3#, 4# centrifuges share a brake unit to form a group, and when both groups are normal, they can be connected in parallel. At the same time, it is also based on the work sequence of the on-site operators. 1# and 2# centrifuges do not brake at the same time, and 3# and 4# centrifuges do not brake at the same time. In normal operation, two centrifuges 1#, 3# are a group, 2#, 4# are a group, and four centrifuges generally do not brake at the same time. Due to the complex environment of the actual work site, the power grid is often shaken and high-order harmonics occur. It can also be used to increase the power supply impedance and help absorb the surge voltage and the voltage spike of the main power supply generated when the nearby equipment is put into operation, so as to ultimately maintain the rectifier unit of the inverter. Each inverter can also use incoming line reactors to effectively prevent the influence of these factors on the inverter. In the renovation of this project, since the original equipment was not equipped with incoming line reactors, the incoming line reactors and other harmonic control devices were not drawn.

Figure 3 Schematic diagram of the inverter and braking unit system after the transformation

3.2 The control system of the control system is shown in Figure 4. After the four inverters are powered on and each inverter is ready for operation, set the output option of the inverter fault relay output terminal to "inverter ready for operation", only the inverter It can be connected in parallel after it is powered on and normal. If any one of them is faulty, the DC bus contactor will not be closed. The inverter fault relay output terminals TA and TC are normally open contacts. After the inverter is powered on, the inverter is "ready for operation", the TA and TC of each inverter are closed, and the DC bus parallel contactor is closed in turn. Otherwise, the contactor will open.

Figure 4 The modified parallel control principle diagram of the braking unit

3.3 Features of the program

(1) Use a complete inverter instead of a simple rectifier bridge plus multiple inverters.

(2) There is no need to have a separate rectifier bridge, charging unit, capacitor bank and inverter.

(3) Each inverter can be separated from the DC bus separately without affecting other systems.

(4) Control the connection of the inverter's DC shared bus through the interlock contactor.

(5) Interlock control to protect the capacitor unit of the inverter hanging on the DC bus.

(6) All inverters connected to the bus must use the same three-phase power supply.

(7) After the inverter fails, quickly disconnect from the DC bus to further reduce the inverter fault range.

3.4 Inverter main parameter setting

Run command channel selection F0.03=1 Maximum operating frequency setting F0.05=50 Acceleration time 1 setting F0.10=300 Deceleration time 1 setting F0.11=300 Fault relay output selection F7.12=15 AO1 output Function F7.26=2 3.5 Test data after the transformation Incoming line voltage when stopping: 3PH 380VAC Bus voltage: 530VDC DC bus voltage: 650V When one set speeds up, the bus voltage drops, and the other set speeds down at this time, the DC bus The voltage fluctuates between 540V and 670V, and the braking unit is not turned on at this time. The DC voltage of the braking unit generally works is 680V as shown in Figure 5 for test analysis.

Fig. 5 The working current monitoring diagram of the modified braking unit

4. Energy-saving analysis

The feedback braking unit itself is an energy-saving application compared to the resistance energy consumption braking, but it is required that each inverter needs to be equipped with a braking unit when it needs to be braked. It is inevitably required that a few inverters must be equipped with several braking units, and the price of the braking unit is not much different from that of the inverter, and the work continuity rate is not very high. The wide application of shared DC bus frequency converter drives in centrifuges can better solve the problem of "one can’t eat enough, and the other can vomit" when one frequency converter increases speed and the other frequency converter brakes. The scheme reduces the repeated setting of the braking unit, reduces the number of work, and also reduces the number of disturbances to the power grid, and improves the power quality of the power grid. It is of great significance in reducing equipment investment, increasing equipment utilization, saving equipment and energy saving.

5, concluding remarks

The widespread application of common frequency converters sharing the DC bus can better solve the problem of non-synchronization of power consumption and power feedback time period. It is of special significance for reducing equipment investment, reducing grid interference and improving equipment utilization.

References

[1] Emerson EV2000 Inverter Chinese Technical Manual. Emerson Network Power Co., Ltd.

[2] User manual of IPC electric energy feedback and resistance braking unit. Shenzhen Jia Neng Company.

[3] Du Jincheng. Electric frequency conversion speed regulation design technology [M]. Beijing: China Electric Power Press, 2002.

[4] Zhong Mingzhen, Zhao Xiangbin. Low-voltage inverter application manual [M]. Beijing: Machinery Industry Press, 2009.

About the Author

Bai Xiangang (1979-) Male, major in electrical engineering and automation, currently working in the electrical management of Sinopharm Group Weisheng Pharmaceutical (Shijiazhuang) Co., Ltd.